How To Fit An External Input to the Juno-106 Chorus Unit

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How To Fit An External Input to the Juno-106 Chorus Unit

Post by HideawayStudio » Fri Jul 02, 2010 12:20 am

Ever Wondered What That Lush Chorus Sounds Like on The Juno-106 When Fed With An External Input?

I recently had to service my Juno-106 and on revisiting the schematics I noticed to my delight that there is an obvious modification almost begging to be made in this beasty. It turns out that the summing amp used to mixed all 6 of the voice module outputs features an extra input in the form of a test point. This test point is clearly there to permit a service engineer both safely monitor and inject signals at this point. The beauty of this input is that its mixed with any signal from the modules and passed directly onto the BBDL based stereo chorus module located on the jack board.

Test Point TP2 on the Module Board Schematic:
Image

On poking around I discovered that it's dead easy to reassign the almost never used 1/4" "Patch Shift" socket on the rear panel to be an audio input.

I had a go making my proposed modification a number of weeks ago and it works a treat!

DISCLAIMER: I TAKE NO RESPONSIBILITY FOR ANY DAMAGE TO AN INSTRUMENT OR THE USER WHILST ATTEMPTING TO REPLICATE THIS PROCEDURE. THERE ARE EXPOSED LIVE MAINS POTENTIALS TO BE FOUND INSIDE THE JUNO-106 POWER SUPPLY.

To Open the Juno-106:

Work on a solid flat surface
Open the Juno-106 by removing the three lower screws on each side cheek. Do not remove the upper screw.
Gently swing open the top panel about its hinge on the rear panel

The External Chorus Mod Procedure:

Locate the jack board mounted in the rear of the left hand side of the top panel
Undo the 2 screws holding it to the rear panel and the three screws to the top of the panel
Gently move the pcb away from the rear panel to expose the underside of the pcb (see picture)

Image

Locate the thin track that runs from the centre pin of socket JA5 "Patch Shift"
Using a sharp knife very carefully cut the track about 1 inch away from where it exits JA5 (see picture)

Image

Solder one end of a good quality non-polarised 1uF capacitor to the centre pin pad of JA5
Prepare a length of miniature shielded cable such that the screen is insulated with heatshrink and the centre core is soldered to the other end of the cap (see picture)

Image

Secure cap with either a blob of hot melt glue or a small double sided sticky pad
Replace the jack board making sure that the new cable is routed to the left out of the way and not trapped
Locate TP2 on the left hand side of the module board (see picture)

Image

Prepare the end of the cable such that the centre core and shield are separated and insulated with heat shrink
Tin the core and shielded portions of the prepared cable end
Solder the centre core to TP2 (see picture above)
Solder the shield to the AN (analog ground) test point about 1 inch to the right of TP2 (see picture)

Image

Carefully route the new cable alongside the existing wiring loom and hold in place with cable ties
The modification is complete

Testing:

Make sure all cropped wire and solder residue is cleared away
Close top lid
Power unit up and check for normal synth operation
Apply line level signal to the Patch Change socket
Check for presence of signal from normal Juno-106 output jacks
The chorus mode can be switched as normal using the OFF - I - II switches

Warning:

Go carefully - never apply excessive signal inputs to the unit ie. only use near line levels. If distortion is heard then lower the input signal.

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by schmidtc » Fri Jul 02, 2010 7:04 am

Nice work!! I'll might just have to try this. It looks as if this is basically an input for voice number 7 ;) Nice how it comes in right before a unity inverting buffer so you don't have to worry about impedance. How does line level audio compare with the other channels? Why the cap? DC blocking?

You might want to keep the patch shift capability to boot. If you try triggering the patch shift with an LFO, it can clock into audio rates oscillating between 3 patches and make sounds most people don't associate with a 106!

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by adamstan » Fri Jul 02, 2010 9:25 am

You might want to keep the patch shift capability to boot. If you try triggering the patch shift with an LFO, it can clock into audio rates oscillating between 3 patches and make sounds most people don't associate with a 106!
I'm afraid that Juno CPU may bee too slow to make such rapid patch changes...I suppose it would either crash, freeze, or (if the OS is written well) just change patches slower missing some trigger signals.
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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by schmidtc » Fri Jul 02, 2010 4:50 pm

That's a good point. I've never turned up the frequency to high (maybe 20 or 30 Hz) because I was also afraid of the CPU stumbling over itself. Caution advised.

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by space6oy » Fri Jul 02, 2010 6:22 pm

bravo, impressive find & great job of sharing it, thanks!
:agree:

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by garranimal » Sat Jul 03, 2010 3:00 am

ooh I'm tempted to do this, too. Sacrificing the patch shift jack isn't the only option, an input jack could be added.
Looking at the schematic a little more, the external signal could be injected on the other side of R1 and not be attenuated by the resistor?

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by guillermotin » Sat Jul 03, 2010 10:22 pm

This post almost made me cry.

As I write, my Juno is already being opened.

Thank you!!

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by HideawayStudio » Sun Jul 04, 2010 9:08 am

garranimal wrote:Looking at the schematic a little more, the external signal could be injected on the other side of R1 and not be attenuated by the resistor?
Arrggh! NO! This is a virtual ground type summation circuit - the signal must never be applied directly to the op-amp's input pins - and it is essential the signal is on the far side of the series resistor R1 so that it can do its balancing act.

If you look at the schematic you will notice that the output of each voice module is stood off with a 33K series resistor in the same manner albeit at a higher attenuation.

If you need reading up on the subject see the wikipedia entry:

http://en.wikipedia.org/wiki/Virtual_ground

An extra jack could be added but I couldn't bare the thought of drilling a hole in my 106 and not being able to remove the mod without trace if I wished.
Last edited by HideawayStudio on Tue Oct 09, 2012 11:44 pm, edited 1 time in total.

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by garranimal » Sun Jul 04, 2010 7:59 pm

HideawayStudio wrote:
garranimal wrote:Looking at the schematic a little more, the external signal could be injected on the other side of R1 and not be attenuated by the resistor?
Arrggh! NO! This is a virtual ground type summation circuit - the signal must never be applied directly to the op-amp's input pins - and it is essential the signal is on the far side of the series resistor R1 so that it can do its balancing act.
Yikes, thanks for the heads up.

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by HideawayStudio » Wed Aug 18, 2010 9:34 pm

Can someone repost the chorus demo that was lost after the VSE crash on this thread - it was superb!

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by Mooger5 » Tue Aug 24, 2010 5:13 pm

HideawayStudio wrote:Can someone repost the chorus demo that was lost after the VSE crash on this thread - it was superb!
Here: http://mp3upload.ca/music/view/28209

What about JA5 that was grounding the path to the CPU board when not in use, should we be concerned about it?
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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by HideawayStudio » Sun Sep 05, 2010 1:33 am

Mooger5 wrote:
HideawayStudio wrote:Can someone repost the chorus demo that was lost after the VSE crash on this thread - it was superb!
Here: http://mp3upload.ca/music/view/28209

What about JA5 that was grounding the path to the CPU board when not in use, should we be concerned about it?
Many thanks for that Mooger5.

I'm not sure about "concern" but it is true that the original signal to the CPU is higher impedance than I'd like after the mod. If erratic behaviour results then I'd be tempted to tie the CPU end of the cut track to ground with a 1K resistor - put it this way, it can't hurt.

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by jumunius » Tue Jan 10, 2012 8:11 pm

Thanks for the idea -- this is great. I've been investigating it a bit as I've been doing some maintenance on my beleaguered Juno. Messing with the chorus inspired me to investigate what else could be done to make the chorus more flexible. Based on testing, it appears that if you put a smallish resistor in series with a 1Meg pot (used as variable resistor), and wire the two of them in parallel across R8 (2.2M) on the jack board, you get faster LFO mod (and a little wider amplitude). This makes for a rich syrupy chorus. Obviously one would want a switch to turn it off, since the existing chorus is nice too. I'm planning on just using a switched pot.

I'm not knowledgeable enough to know whether or not this causes any stability problems, but I'm reckless enough to try it and it sounds really good.

Another thought I had is that you could potentially use the chorus LFOs for other purposes. So that could mean connecting the TP3 & 4 to any combination of the 4.7k resistors on the other side of the panel board's sliders. I haven't tried this yet though, and maybe I'd want to use a DPDT switch to disable the send to chorus when doing this. I think they are inverted or phase shifted from one another, so sending them to two independent parameters might produce interesting results. They also have a somewhat rounded (slewed) square shape which in itself would be a nice addition.

I do know that you can use those 4.7k resistors for inputting CVs. If you want an easy way to add an external CV in, you can put it into any one of those (maybe you want a rotary switch to give you a number of routing options). Even easier is to route an external CV to the wiper of the bender, or to the other end of R12, depending on whether or not you employ a switch to disable the bender when an external CV source is plugged in.

All of these are work-in-progress ideas, and most of them would entail defacing your Juno in unspeakable ways. Most of them are tested enough to confirm that they work and sound good, but if anyone has any idea as to why not to try them I'm all ears.

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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by syntheticsolutions » Tue Oct 09, 2012 12:39 am

Does anyone know if this would be a possible mod for the Juno 6?

It would be such an amazing feature on my 6
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Re: How To Fit An External Input to the Juno-106 Chorus Unit

Post by Bitexion » Tue Oct 09, 2012 12:50 am

The easier way to get the Roland chorus on other units is to buy a Roland/Boss CE-300 "super chorus" unit.
It's the same circuit as the juno chorus, only in a separate box. It's a 1U rack unit. Plus, you get loads of extra controls like depth, speed etc.

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